The present invention relates to new aluminium polychlorosulphates having high basicity and stability, low sulphate content, and high coagulating and flocculating power, and to an improved production process thereof, easy to perform, carried out at room temperature, and free from any contamination and residue.
Aluminium polychlorosulphates are used in drinking and waste water treatment, in paper industry and in many industrial clarification processes. The performance and the applications of these aluminium polychlorosulphates depend not only on their chemical characteristics, but in particular on the process used for their manufacture. In most different fields of application it has been observed that aluminium polychlorosulphates with a very similar empirical formula can present completely different performances, depending on the method used for their preparation. The reason of this behaviour is attributed to the presence in solution of ionic polycondensed species, whose constitution and structure depend on the manufacturing process.
To the several applications of aluminium polychlorides and polychlorosulphates belong the manufacture of antiperspirant compositions, the production of catalyst supports, the sizing treatment of paper in paper industry and the treatment of water and waste water.
In water and waste water treatment the aluminium polychlorides and polychlorosulphates have been gaining strength to the detriment of other traditionally used compounds, like iron salts and aluminium sulphate, owing to the greater coagulating and flocculating power of these basic aluminium salts. The coagulating and flocculating properties can be improved by addition of sulphate ions (SO42xe2x88x92), whose synergy is well known since a long time.
Unlike aluminium sulphate solutions which are very stable, aluminium polychlorosulphate solutions are stable only in certain conditions of concentration, basicity and sulphate content.
The aluminium polychlorosulphates suitable for water treatment, especially for the treatment of drinking water, must exhibit a sufficiently high basicity and a sufficiently low sulphate content to guarantee a low level of residual aluminium in the treated water. On the other hand, the level of sulphate must be sufficiently high to guarantee good coagulating/flocculating properties. Too high basicities impair the coagulating/flocculating power. Low basification temperatures promote a good coagulation/flocculation. High basicities associated with high levels of sulphate impair the stability of aluminium polychlorosulphates.
In order to obtain aluminium polychlorosulphates suitable for water treatment, especially for drinking water treatment, a balance must be found between a sufficiently high level of sulphate to favour good coagulating/flocculating properties, without increasing the soluble aluminium content in treated water, and a sufficiently high basicity to guarantee low levels of residual aluminium, without impairing the coagulating/flocculating properties. Further the product must be stable, the preparation process must be simple and must not release any type of pollutant.
Aluminium polychlorosulphates that satisfy some of the above-mentioned requirements are known:
a) The French Patent Application No. FR-A-2317227 describes the process, carried out at a temperature lower than 50xc2x0 C., for the preparation of aluminium hydroxychloride having the general formula:
Al(OH)aClbYc/z1Md/z2
where:
Y represents an anion with valency Z1, typically SO42xe2x88x92
M represents a cation with valency Z2, such as ammonium, an alkali metal or an alkaline-earth metal and
1.2 less than a less than 1.7,
0 less than c less than 0.6,
0.2 less than d less than 1.7, and
a+b+c=3+d.
Nevertheless the basicities of above-mentioned aluminium polychlorides are lower than 57% and not high enough for some applications, as in water treatment.
b) The Japanese Patent No. JP 52113384 describes a process for the preparation of aluminium polychlorosulphate having high basicity in which an alkaline reagent, such as CaCO3, NaHCO3, Na2CO3, Mg(OH)2 or MgO, is added at a temperature lower than 60xc2x0 C. to a solution of an aluminium compound previously prepared by reacting a low basicity aluminium polychlorosulphate with CaCO3, thus obtaining gypsum as by-product, which is then separated. The major drawbacks of this process are the following: the basicity of this intermediate aluminium compound is 50%, the residual aluminium levels are too high and a considerable production as by-product of gypsum, which is of difficult disposal, occurs.
c) The Japanese Patent No. JP 53001699 describes a process for the preparation of high basicity aluminium polychlorosulphate, where first a medium basicity aluminium polychlorosulphate is reacted with CaCO3 in equimolecular amount with respect to sulphate, thus obtaining gypsum, which is then separated. In the next step, the so obtained product with a basicity ranging between 55% and 58%, is reacted with a compound chosen among CaCO3, NaHCO3, Na2CO3, Mg(OH)2 and MgO.
Nevertheless, even if these processes lead to high basicity compounds (basicity lower than 70%), they present the major drawback of a large acidic gypsum coproduction, of difficult disposal.
d) The European Patent No. EP-327419 describes a process for the preparation of aluminium polychlorosulphate having the general formula
AlnClm(OH)3n+2Kxe2x88x92mxe2x88x922p(SO4)pXk
where
X represents an alkaline-earth metal, preferably calcium,
n, m, p, q represent the molar concentrations (mole/litre) of the ions in solution, [(3n+2Kxe2x88x92mxe2x88x922p)/3n]xc2x7100 represents the basicity.
The basicity of the products described in EP-327419 ranges between 45% and 70%, thus being high enough, and the products satisfy the above described requirements for water treatment; nevertheless, they present the major drawback of generating acidic gypsum as a by-product, whose disposal represents an ecological problem difficult to solve. Furthermore, the production process is complicated, including a filtration step, the washing of the filter cake and the recovery of active principle. A cooking phase at high temperature is also necessary. The investment and manufacturing costs are therefore elevated.
e) The European Patent No. EP-0557153 describes a process for the preparation of aluminium polychlorosulphates with a high basicity ranging between 65% and 75%, and low effluent production. Nevertheless a big disadvantage of this process is its scarce reproducibility. This lack of reproducibility is shown by the presence of high levels of residual aluminium in treated water in 80% of cases.
f) The European Patent No. EP-0794153 describes aluminium polychlorosulphates having the general formula:
Al(OH)aClbYc/dMxe2x80x2Cl2/eMxe2x80x3Cl
where
Y represents an anion with valency 2, such as SO42xe2x88x92,
Mxe2x80x2 represents a cation with valency 2, such as an alkaline-earth metal,
Mxe2x80x3 represents a cation with valency 1, such as an alkali metal or ammonium,
a, b, c, d are non-zero positive integers or non-zero positive fractions,
e can be zero or a positive integer or a positive fraction,
where:
1.8 less than a less than 2.25
0.001 less than c less than 0.15
b=3xe2x88x922cxe2x88x92a
2d+e+bxe2x89xa63.
The process described in EP-0794153 reduces but does not eliminate the co-production of gypsum illustrated in the above mentioned patent EP-327419. The process is complicated because it includes a filtering operation and a cooking step at high temperature.
On the other hand, according to the above-mentioned process, the use of an alkaline-earth metal compound in the basification step is necessary, which can cause the precipitation of alkaline-earth metals sulphates, typically of calcium, scarcely soluble in an aqueous salt solution.
Now the Applicant has found an advantageous process for the preparation of new aluminium polychlorosulphates having high basicity and stability, a low sulphate content, and a high flocculating and coagulating power, corresponding to the following general formula (I)
Al(OH)lClm(SO4)nMpxe2x80x83xe2x80x83(I)
where
M represents an alkali metal,
l, m, n, p represent the number of moles per mole of aluminium, so that
1.74xe2x89xa6lxe2x89xa62.25,
0.01xe2x89xa6nxe2x89xa60.17,
0.32xe2x89xa6pxe2x89xa61.49, and
l+m+2n=p+3.
These compounds may be obtained by a process comprising the following steps, carried out at room temperature:
(a) reaction with a basic compound of an alkali metal, of a aluminium polychloride or polychlorosulphate having the general formula (Ixe2x80x2)
Al(OH)lxe2x80x2Clmxe2x80x2(SO4)nMpxe2x80x2xe2x80x83xe2x80x83(Ixe2x80x2)
xe2x80x83where
1.1xe2x89xa6Ixe2x80x2xe2x89xa61.44,
0xe2x89xa6nxe2x80x2xe2x89xa60.10,
pxe2x80x2xe2x89xa6p (p of formula (I)),
Ixe2x80x2+mxe2x80x2+2nxe2x80x2=pxe2x80x2+3;
(b) reaction of the product obtained from step (a) with a compound that provides sulphate ions.
The subjects of the present invention are therefore the above aluminium polychlorosulphates of formula (I) and the preparation process thereof. The present compounds of general formula (I) have proved effective as coagulation and flocculation agents; the use of the present compounds as coagulation and/or flocculation agents for example in water treatment processes, in the paper industry, in the industrial clarification processes, in the preparation of catalyst supports and of antiperspirant compounds, constitutes a further subject of the present invention.
The present process allows to prepare new aluminium polychlorosulphates of high basicity, low sulphate content, high coagulating and flocculating power and good stability, without the co-production of gypsum and of any type of effluent, and with low level of residual aluminium in treated water.
The present process is easy to perform because no heating, cooling nor filtering of the process streams is required. The co-production of gypsum or of any other effluent is completely eliminated.
The basicity of the present formula (I) compounds may range from 58 to 75%, preferably from 58 to 66%, where the basicity is defined by the formula
B %=(l/3)xc2x7100.
The present process for obtaining the compounds of formula (I) comprises the reaction at room temperature of an aluminium polychloride or polychlorosulphate, having the general formula(Ixe2x80x2)
Al(OH)lxe2x80x2Clmxe2x80x2(SO4)nxe2x80x2Mpxe2x80x2xe2x80x83xe2x80x83(Ixe2x80x2)
where
1.1xe2x89xa6Ixe2x80x2xe2x89xa61.44,
0xe2x89xa6nxe2x80x2xe2x89xa60.10,
pxe2x80x2xe2x89xa6p (p of formula (I)),
Ixe2x80x2+mxe2x80x2+2nxe2x80x2=pxe2x80x2+3,
with a basic compound of an alkali metal, until the reaction is completed.
Subsequently sulphate in form of an alkali metal sulphate or sulphuric acid is added.
This is followed by a conditioning step of 1 to 5 hours, preferably 1 to 3 hours, at room temperature.
The aluminium polychlorides and polychlorosulfates which can be used according to the invention are well known and obtainable by reacting hydrated alumina with hydrochloric acid or a mixture of hydrochloric and sulphuric acid at high temperature. Such products are generally available as aqueous solutions or to a lesser extent as solid products under various tradenames such as Prodefloc(copyright) (polyaluminum chloride; Al2O3 16-18%; basicity 38-42%) from the firm Prodeco or Wac(copyright) (basic aluminum polychlorosulphate aqueous solution; Al2O3xe2x89xa710%; basicity xe2x89xa738.5%) from the firm Elf Atochem or Panfloc(copyright) (polyaluminum chloride; Al2O3 16-18%; basicity 38-42%) from the firm Panamericana.
The basic compounds of an alkali metal that can be used in the reaction with the compound of formula (I) are, for example, Na2CO3, NaHCO3, NaOH, K2CO3, KHCO3 and KOH. Preferably Na2CO3 or NaOH are used. Such compounds can be used as such, or optionally may be used in mixture with basic alkaline-earth metal compounds, depending on the solubility of the alkaline earth metal sulphates in the reaction medium.
Alkaline-earth metal basic compounds of possible use according to the present invention are, for example, CaO, Ca(OH)2, CaCO3, MgO, Mg(OH)2, MgCO3, CaHCO3, MgHCO3.
The sulphate ions can be provided by Na2SO4, NaHSO4, Na2SO4xc2x710 H2O, K2SO4, KHSO4 and H2SO4, preferably Na2SO4 or H2SO4 are used.
The basic compounds of alkali metals and, in case, alkaline-earth metals, and the compounds containing sulphates can be used in the form of solid salts or aqueous solutions.
As shown in the general formula above, the aluminium polychlorides or polychlorosulphates used as starting material may contain or not cations M, which can originate from the raw materials used to prepare the above-mentioned aluminium compounds or from a partial neutralisation of the above-mentioned aluminium compounds with a basic compound containing a cation M; the starting material can also be a aluminium polychlorosulphate instead of an aluminium polychloride, with a sulphate level nxe2x80x2 less than 0.10. This way, in a second step, the corresponding amount of sulphate up to a level of n=0.17 may be complemented, according to the invention.
According to the present invention the introduction order of the reactants into the reaction medium is so that first the basification with a basic compound of an alkali metal is accomplished. Once completed the basification reaction, the compound containing sulphate is added.
A conditioning step under gentle stirring at room temperature follows, preferably with a duration ranging between 1 and 3 hours.